Process Boiler #1 Non-condensing Stack Economizer Requirement
Measure Overview
This proposal would require boiler stack (non-condensing) economizers on process boilers with capacities at or above 10 MMBtu/hr. The requirement would apply to all new process boilers, including replacement boilers and boilers in additions to existing facilities, with the following exceptions:
- Boilers with stack temperatures below 340°F at their lowest firing rate without an economizer.
- Boilers that burn biofuels or hydrogen.
- Boilers employing other methods of stack heat recovery, such as a heat exchanger that serves an industrial heat pump or process drying application.
The Statewide CASE Team is evaluating one additional potential exemption for the requirement:
- Sites where the retrofit building ceiling height or existing footprint shape results in insufficient room to install a standard boiler plus stack economizer combination.
In addition, this proposal would adjust the 2025 Title 24 Part 6 code language which calls for qualifying process boilers to maintain excess oxygen concentrations at 3.0 percent or lower over firing rates of 20 to 100 percent. The proposed code changes would update the code language to allow excess oxygen concentrations as follows:
- ultra-low NOx burners: maximum 7% excess O2 at or above 20% of maximum fire
- mesh burners: maximum 10% excess O2 at or above 20% of maximum fire
- boilers equipped with Selective Catalytic Reduction (SCR) systems: maximum 4% excess O2 at or above 25% of maximum fire
Boilers used in oilfield production and utility and municipal power generation are not expected to be within the scope of Title 24, Part 6.
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The Statewide Utility Codes and Standards Team is conducting a survey to inform nonresidential code change proposals for the 2028 California Building Efficiency Standards (Title 24, Part 6).
In appreciation, we will send you a $20 Tango gift card or donate this amount to your local United Way upon completion of the survey.
Take the SurveyScope of Proposed Code Change
| Building Type(s) | Construction Type(s) | Type of Change | Updates to Compliance Software | Third Party Verification |
| Nonresidential | New construction, additions, alterations | Mandatory | No updates | Add new verification requirements |
Justification for Proposed Change
Most steam boilers lose 20% or more of their input fuel energy in the form of combustion exhaust. A stack economizer, which is an assembly of finned tubing placed in the boiler exhaust stream, can recover a significant fraction of this waste heat by pre-heating the boiler feedwater with the hot boiler flue gas. This reduces load and thermal stress on the boiler, lowers fuel consumption, and extends boiler useful life.
The use of boiler stack economizers has been listed in Department of Energy literature as a best practice since at least the early 2000s. Despite its cost-effectiveness, this measure often goes unimplemented because of its first cost and the general lack of owner and operator awareness of the energy benefits. In general, boiler system vendors and contractors are highly familiar with this technology, and it is widely available. Stack economizers can be installed in various configurations, including roof boxes, making them feasible in most facilities.
The use of a stack economizer is typically the highest-impact action that sites can take to reduce their boiler natural gas usage, saving 2-3% of fuel energy by preheating boiler feedwater. Preliminary statewide natural gas savings from a stack economizer requirement are estimated at 1.37 million therms per year. Preliminary savings calculations and assumptions can be found in the stack economizer savings calculations document.
2025 Title 24 Part 6 requirements dictate that qualified boilers shall maintain stack-gas oxygen concentrations lower than or equal to 3.0 percent over firing rates of 20 to 100 percent. However, boilers that comply with California’s local air quality district NOx emission rate limits, require higher amounts of excess oxygen than non-compliant, higher-Nox boilers—often above 3.0 percent. Updating Title 24 Part 6 requirements to allow for oxygen concentrations higher than 3.0 percent would allow for boilers to comply with California’s local air quality district requirements. The proposed requirements would account for different boiler types that require different oxygen concentrations to maintain efficiency. Eliminating conflicting regulations through this proposal would improve ongoing code compliance. In addition, NOx monitoring often drives utilization of a control monitoring console, which enhances data utilization to improve decision-making and is synergetic with the monitoring requirements in this proposal.
Additional benefits of this proposed code change include job creation in the manufacturing and installation of stack economizers, as well as improved local air quality. Many industrial facilities are located near Low- and Moderate-Income (LMI) housing, which is disproportionately exposed to lower air quality. This proposal would reduce photochemical smog in these communities.
Relevant Documents
Round One Utility-Sponsored Stakeholder Meeting Materials
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The Statewide CASE Team values input from all stakeholders engaged in the Title 24, Part 6 code change process. We encourage the open exchange of code change comments and concerns.